This invention relates to compositions, containing certain N-phenylmethylalkenamides, having analgesic activity.
While "pain" is incapable of precise definition due to its basically subjective nature, it can generally be said that the term refers to feelings of distress or suffering caused by stimulation of specialized nerve endings. A great variety of drugs have been developed to reduce pain in man and other animals; some directed to eliminating pain at its source, and others directed to blocking the assimilation of pain by the brain. Among the latter group of drugs that are designed to block the sensation of pain, are the analgesics, which generally relieve pain without causing unconsciousness. Analgesics can be further classified in two main categories: opioid analgesics, including morphine, codeine, levorphanol, and the morphine-like analgesics meperidine, and methadone; and antipyretic analgesics, such as aspirin, phenacetin, acetaminophen, phenylbutazone, and indomethacin.
Although the precise pharmacological action of these analgesics is uncertain, there are certain effects which readily distinguish the opioid analgesics from the antipyretics. In particular, the antipyretics are weak analgesics, with much of their effect in the peripheral nervous system, so that behavioral changes do not usually occur. Generally, these analgesics relieve only somatic pain originating from muscles, joints, tendons, and fasciae, and are ineffective against deep visceral pain. However, the opioid analgesics are quite effective against all types of pain, with broad based action in the central nervous system. Aside from potent analgesia, the opioids, also known as narcotics, often produce effects on mood and other behavioral changes. Perhaps the most notable side effect of the opioid analgesics is the fact that their repeated use is associated with tolerance as well as psychic and physical dependence.
It has been recently discovered that capsaicin, a natural product of certain species of the genus Capsicum, induces analgesia in animals. Capsaicin (8-methyl-N-vanillyl-6E-nonenamide) and "synthetic" capsaicin (N-vanillylnonanamide) are disclosed as analgesics in U.S. Pat. No. 4,313,958, LaHann, issued Feb. 2, 1982. Analgesic activity of capsaicin has also been discussed in the chemical and medical literature, including Yaksh, et al., Science, 206, 481-483 (1979). The use of capsaicin to prevent dipilatory irritation is also disclosed in U.S. patent application Ser. No. 330,731, LaHann, et al., filed Dec. 14, 1981.
Specifically, capsaicin prevents the development of cutaneous hyperalgesia and also provides relief of deep visceral pain and severe pain. In certain tests, capsaicin produces a level of analgesia comparable to morphine, yet it is not antagonized by such narcotic antagonists as nalorphine and naloxone. Thus, capsaicin does not appear to belong to either of the major categories of analgesics, described above.
Compounds structurally similar to capsaicin have been described in the chemial literature. These references, though, do not suggest analgesic activity for these compounds. For example, Newman, "Natural and Synthetic Pepper-Flavored Substances (6)," Chemical Products, 102-106 (1954) lists the relative pungency of approximately 164 compounds, including N-vanillyloleamide and other alkenamide derivatives of capsaicin. Ott and Zimmermann, Liebigs Ann., 425, 314-337 (1921) discloses a synthesis for N-vanillyloleamide. A synthesis for N-vanillylinoleamide is disclosed in Ferris, Australian Commonwealth, Dep. Supply, Def. Stand. Lab., No. 89 (1966) (Chem. Abs. 67:22919).
U.S. Pat. No. 4,238,505, Nelson, issued Dec. 9, 1980, discloses 3-hydroxyacetanilide for use in producing analgesia in animals. U.S. patent application Ser. No. 359,464, LaHann, et al., filed Mar. 18, 1982, now U.S. Pat. No. 4,424,206, issued Jan. 3, 1984, describes hydroxyphenylacetamides with analgesic and anti-irritant activity. Similarly, analgesic and anti-irritant activity is disclosed for N-vanillylsulfonamides in U.S. patent application Ser. No. 360,953, Buckwalter, et al., filed Mar. 23, 1982, now U.S. Pat. No. 4,401,663, issued Aug. 30, 1983; N-vanillylureas in U.S. patent application Ser. No. 381,672, Buckwalter, et al., filed May 25, 1982, now U.S. Pat. No. 4,460,602, issued July 17, 1984; and N-vanillylcarbamates in U.S. patent application Ser. No. 384,685, Buckwalter, et al., filed June 3, 1982, now U.S. Pat. No. 4,443,473, issued Apr. 17, 1984.
It has now been discovered that certain N-phenylmethyl-cis-monoalkenamides have analgesic activity in humans and lower animals. In particular, these alkenamides have potent analgesic activity similar to that of capsaicin, but are substantially less toxic.